Editing Optical telescope (section)

==Principles==
{{for|specific designs of telescope|Reflecting telescope|Refracting telescope|Catadioptric}}

The basic scheme is that the primary light-gathering element, the [[objective (optics)|objective]] (1) (the [[convex lens]] or [[concave mirror]] used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a [[real image]] (5). This image may be recorded or viewed through an [[eyepiece]] (2), which acts like a [[magnifying glass]]. The eye (3) then sees an inverted, [[magnification|magnified]] [[virtual image]] (6) of the object.

[[Image:Kepschem.png|thumb|Schematic of a [[Keplerian telescope|Keplerian]] [[refracting telescope]]. The arrow at (4) is a (notional) representation of the original image; the arrow at (5) is the inverted image at the focal plane; the arrow at (6) is the virtual image that forms in the viewer's visual sphere. The red rays produce the midpoint of the arrow; two other sets of rays (each black) produce its head and tail.]]

===Inverted images===
{{Further|Star diagonal|Relay lens}}
Most telescope designs produce an inverted image at the focal plane; these are referred to as ''inverting telescopes''. In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation. In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a mirror diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect, but still reversed left to right. In terrestrial telescopes such as [[spotting scope]]s, [[monocular]]s and [[binoculars]], prisms (e.g., [[Porro prism]]s) or a relay lens between objective and eyepiece are used to correct the image orientation. There are telescope designs that do not present an inverted image such as the [[Refracting telescope#Refracting telescope designs|Galilean refractor]] and the [[Gregorian telescope|Gregorian reflector]]. These are referred to as ''erecting telescopes''.

===Design variants===
Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design ([[Newtonian telescope]], [[Cassegrain reflector]] or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over a larger field of view.
<!--no special reason to mention a Maksutov telescope here, it is a rare type; the Cassegrain is the "grandfather" of all these designs. Also, folding the optical path alone, ''e.g.'' by flat mirrors, should not reduce the field of view-->